How Does the Camera Know How Much Light to Let In?

The amount of light entering the camera and the sensitivity of the medium—whether it be film or an electronic sensor—combine to give what is commonly called exposure. The aperture size and shutter speed control how much light is allowed to enter the camera, while the ISO setting controls the sensitivity of the sensor. The question, though, is how much light is actually needed for the correct exposure? Almost all cameras nowadays have some means of measuring light and suggesting three settings that will allow this amount of light in.

The photographer can then do one of three things with the camera’s suggested choices:

1. Accept the camera’s choice of settings

2. Change more than one of the settings, but keep the exposure the same; for example, double the shutter speed and make the aperture a full stop larger

3. Change the exposure, which will result in the image being lighter or darker than if either of the first two methods were used

Overexposing the image or “blowing” the highlights can really help to focus attention